Sanjay Singhania (Aamir Khan) is a rich tycoon suffering from short term memory loss due to being hit by a metal pole when trying to intervene on his girlfriend's murder. Because of the severe injury on his head, his memory can only last for fifteen minutes and he doesn't remember events or incidents that have happened before in his life. He can now only live a comprehendible life by tattooing notes on himself and taking pictures of things with a Polaroid camera to remind himself of the incidents that have happened. His story in unraveled by a police officer (Riyaz Khan) who is hunting him due to several murders he has committed while trying to find his girlfriend's murderer through his diary. His story has also piqued the interest of a medical college student, Chitra (Jiah Khan) who is searching for a patient with an "interesting" record for her college project. Chitra comes to know about Sanjay's love with Kalpana (Asin Thottumkal) a model cum activist. When her activist work for children comes in the way of child racketeers, she is killed by Ghajini when she intervened with his plan of kidnapping 25 young girls to Goa. So, with the help of Chitra, Sanjay sets out on a revenge spree, searching for the murderer of the one love of his life...